1. Field of the Invention
This invention relates to an automatic transmission control. The control is used in connection with a solenoid valve which accomplishes gear ratio stages. More particularly, the control method is useful for a full automatic electronic controlled transmission.
2. Description of the Prior Art
A conventional electronic controlled transmission is disclosed, for example, in Japanese Laid-Open Publication No. 60-30864 (TOKU-KAI-SYO 60-30864), issued on Feb. 16, 1985.
FIG. 4 of the accompanying drawing shows a conventional type of a solenoid valve 100 for use in such a transmission, the solenoid valve comprising an electromagnetic coil 101, a core 102 and a plunger 103. An electronic controlled transmission is equipped with a plurality of those conventional solenoid valves which establish a plurality of gear ratio stages in connection with an operation of a manual shift valve.
As seen from FIG. 4, there is a clearance 104 between the core 102 and the plunger 103. A spring 105 is provided between the core 102 and the plunger 103 which produces a proper tension to bias the plunger 103 away from the core. An inlet oil conduit 106 is connected to an inlet port 107 of the solenoid valve 100. An outlet oil conduit 108 is formed in the solenoid valve 100.
The above mentioned solenoid valve is a normally closed type of solenoid valve. When the solenoid valve 100 is magnetized, the plunger approaches the core whereupon the inlet conduit 106 and the outlet conduit 108 are communicated with each other. On the other hand, when the solenoid valve 100 is demagnetized, the spring extends the plunger so that the inlet conduit 106 and the outlet conduit 108 are not communicated with each other.
During the operation of the electric controlled transmission, if trash invades the clearance 104 of the solenoid valve 100, the operation of the solenoid valve 100 may be interrupted. This operating condition is called "solenoid-stick" which obstructs the operation of the electronic controlled transmission and can result in the engine overrunning, especially if the transmission had been signalled to down-shift when the solenoid-stick occurred.